In recent years, miniaturization of electronic equipment typified by a personal digital assistant and the like has increasingly advanced, and therefore, connectors for use therein have a significant tendency of having a narrow pitch, a low profile, and a narrow width. Since a smaller connector has a narrower pin width and takes a working shape folded into a small size, a high strength for obtaining required spring properties is required for a member to be used. From this point of view, a copper alloy containing titanium (hereinafter referred to as a “copper-titanium alloy”) has a relatively high strength and is the most excellent in stress relaxation characteristics among copper alloys. Therefore, the copper-titanium alloy has been used for many years as materials for signal system terminals in which strength is particularly required.
The copper-titanium alloy is an age-hardening copper alloy. When a supersaturated solid solution of Ti which is a solute atom is formed by solution treatment, and the supersaturated solid solution is subjected to relatively long-time heat treatment at low temperatures, a modulated structure which is a periodic change of Ti concentration will develop in a matrix phase by spinodal decomposition, thereby improving strength. At this time, there is a problem in that strength and bending workability are mutually contradictory properties. That is, when strength is improved, bending workability will be impaired, and conversely, when bending workability is emphasized, desired strength will not be obtained. Generally, as the rolling reduction ratio in cold rolling is increased, dislocation introduced is increased to thereby increase dislocation density, which increases nucleation sites contributing to the precipitation and can increase the strength after aging treatment. However, if the rolling reduction ratio is excessively increased, bending workability will deteriorate. Therefore, it has been an object to achieve coexistence of strength and bending workability.
Thus, techniques for achieving coexistence of strength and bending workability of a copper-titanium alloy have been proposed from the point of view of adding a third element such as Fe, Co, Ni, and Si (Patent Literature 1), controlling the concentration of impurity elements which form a solid solution in a matrix phase and precipitating the impurity elements in a predetermined distribution form as second phase particles (Cu—Ti—X-based particles) to increase the regularity of modulated structure (Patent Literature 2), specifying elements to be added in a trace amount which are effective in forming fine crystal grains and the density of second phase particles (Patent Literature 3), forming fine crystal grains (Patent Literature 4), controlling the crystal orientation (Patent Literature 5), and the like.
Further, Patent Literature 6 discloses that as the modulated structure of titanium resulting from spinodal decomposition develops, the amplitude (difference between the highest concentration and the lowest concentration) of the change of titanium concentration increases, thereby imparting toughness to a copper-titanium alloy to improve strength and bending workability. Then, Patent Literature 6 proposes a technique of controlling the amplitude of Ti concentration in a matrix phase resulting from spinodal decomposition. Patent Literature 6 discloses that heat treatment (sub-aging treatment) is performed after final solution treatment to cause spinodal decomposition in advance, followed by performing a conventional cold rolling and a conventional aging treatment or aging treatment at a lower temperature for a shorter time than the conventional conditions, thereby increasing the amplitude of Ti concentration to achieve an increase in the strength of copper-titanium alloy.